ARCHIVED REPORTS XR0009884 - Laserfiche WebLink

Home Browse Search

Report—Remedial Action Feasibility Page 20 Cutter Lumber—Western Lift Site GPE Project No 102 3 August 1,2003 9.0 FEASIBILITY OF CORRECTIVE ALTERNATIVES 9.1 Natural Attenuation ' Quarterly groundwater monitoring has been conducted at this release site since early 1998 Examination of the groundwater monitoring data suggests that natural attenuation is degrading the contaminant plume albeit at a very slow rate Data from the peripheral wells (i e MW-8, MW-9, MW-10, & MW-11) seem to suggest that the total "footprint" or areal extent of the contaminant plume is decreasing A log-normal cross-plot of TPH- G concentration against the time (in years) since the wells were constructed was ' generated using Excel software A copy of the cross-plot is included as Cross-Plot 4 A projection was made for each of the three plotted wells as shown in the figure In the core of the contaminated area as represented by samples from MW-6, MW-7, and MW-8B, the Irate of natural attenuation seems to be much slower than that observed in the peripheral wells Over all, contaminant concentrations in these wells have decreased about 50 percent since the wells were constructed The well with the highest absolute concentration (MW-7) has, also exhibited the most rapid decline in contaminant concentration, apparently proportionally high levels of volatiles being destroyed The decline in this well has shown a marked decline in the rate of decrease in recent years The second-most impacted well, MW-8B has shown essentially no decline in contaminant concentration over the life of the project and MW-6 has exhibited a very • modest decline Taken as a whole, the data appears to suggest that it may take several decades to adequately mitigate the release if natural attenuation is used as the sole mitigation method While the data are not in close agreement it is clear that the time needed to for natural attenuation is unacceptably long Based on the rate of attenuation exhibited in the groundwater to date, and projecting forward, it appear that it may take at least an additional 25 for natural attenuation to progress sufficiently for a site closure to ' be considered possible Should the natural attenuation method of site mitigation be employed, no additional work ' would be completed except for continued groundwater monitoring It is likely that over this period of time it would be necessary to repair or replace some, or all, of these wells Assuming that groundwater monitoring costs $12,000 per year in constant dollars, the total cost of monitoring would be more than $300,000 1 9.2 Soil Vapor Extraction The pilot vapor extraction test documented by this report clearly show that vapor extraction as a stand-alone corrective measure will not be effective in mitigating this site This is due to two primary reasons First, the vadose zone soil is so poorly transmissive, that vapor extraction would require very tight well spacing to achieve pneumatic contact with the entire impacted vadose zone Secondly, with the bulk of the contaminant mass lying 5 to 15 feet below the groundwater surface, vapor extraction would be unlikely to affect the deeper contamination under the best of circumstances The radius of influence 2937 Veneman Ave, ##11240 Geo-Phase Environmental Inc Phone(209) 569-0293 Modesto,CA 95356 Fax(209)569-0295